(1) Field of the Invention
This invention pertains to a fluid operated control system for a variable displacement pump or pumps driven by a prime mover such as an internal combustion engine. More particularly, the invention is directed to a system for controlling the per cycle displacement of a variable displacement pump or pumps supplying fluid under pressure to implements actuators in which the torque requirement of the pump or pumps can be varied without changing the setting output condition of the prime mover.
(2) Description of the Prior Art
In a conventional control of a variable displacement pump (which will be hereinafter referred simply to as a variable pump), a device for controlling displacement of the variable pump is known for example, wherein a discharge oil of a control hydraulic pump is supplied through a control valve to a servo cylinder for changing a swash plate angle of the variable pump, and a pressure reducing operation of the control valve is controlled according to a discharge pressure of the variable pump, thus controlling displacement of the variable pump according to the discharge pressure thereof and maintaining a torque requirement or torque demand of the variable pump (displacement per cycle of pump x pressure) constant. Namely, a control device defining a self pressure as a control signal is known.
In such a control device as above, because the torque requirement of the variable pump is constant, it is common that the torque requirement is set to a torque requirement corresponding to a rated point under a maximum set output condition (full load) of an engine for purpose of effective utilization of engine horse power, and the torque requirement of the variable pump is dependent upon set output conditions of the engine, that is, lever positions of a fuel injection pump of the engine.
Further, when the set output condition of the engine is set to a partial load, that is, the lever position of the fuel injection pump is set to a low speed side to reduce a set output, a rotational speed of the engine is reduced, but the torque requirement of the variable pump is not temporarily changed. However, as a rotational speed of the variable pump is reduced, the torque requirement of the variable pump is resultantly reduced to decrease displacement per unit time of the variable pump. Therefore, an operating speed of implement actuators is reduced. For example, in a constructional machine such as a power shovel, when loading work of light-weight materials and ground levelling work are carried out, it is necessary to quickly operate implements with no need for large power. In such light work as above, if the engine is driven at low speeds, displacement per unit time of the variable pump is reduced as mentioned above, resulting in reduction in operating speed of the implement actuators and reduction in working efficiency. On the other hand, in such a partial loaded condition of a set engine output as above, a maximum torque of the engine is rendered lower than a rated torque under full load, and accordingly the torque of the engine is rendered lower than the torque requirement of the variable pump, resulting in the possibility of engine stall.
Accordingly, in the case that the engine is driven where air density is small or a crude fuel is used as engine fuel, the engine output corresponding to the lever position may not be obtained. Therefore the torque corresponding to the rated torque may not be obtained in spite of setting of the engine under full load. As a result, the torque requirement of the variable pump with respect to an effective torque of the engine is enlarged to disadvantageously decrease the engine rotational speed, and in the worst case, to stop the engine. To avoid these disadvantages, when the engine is set at a full loaded rotational speed so as to sufficiently secure displacement per unit time of the variable pump, fuel consumption of the engine is uneconomically increased instead.